I watch my CI pipeline spend a minute or two creating the virtualenv and pip install into it several times a day, despite the fact that nothing has changed (no change to requirements.txt, etc), and I wish there was a well-supported way to package up a venv into some kind of artifact (keyed by Python version, host platform and package versions) so we only had to rebuild it if something changed. This sounds a bit like that, but also sounds like something more complicated than what I was looking for.
Is it re-solving for dependencies every time? Have you checked where the bottleneck is? (Don't CI pipeline tools hold onto Pip's download cache? Maybe this is part of why Setuptools gets so many downloads...)
